Previously we developed radioimmunoassays for the MM and BB isoenzymes of creatine kinase in human serum. Currently we are investigating the clinial usefulness of these assays by measuring isoenzymes levels in serum and cerebrospinal fluid from patients with schizophrenia, in the serum, pleural fluid and prostatic fluid of patients with neuromuscular diseases, prostatic and other cancers, myocardial injury, and in the serum of patients undergoing cardiac surgery. The assay for the BB isoenzyme seems particularly useful. Elevations in CK-BB occur in neuromuscular diseases, myocardial infarction, prostatic and several other cancers, as well as following cardiac surgery. The use of serum CK-BB as a tumor marker in patients with prostatic and other carcinomas appears particularly promising.